This invention relates to a developing device for supplying a developing agent to an electrostatic latent image formed on an image carrier and developing the image.
Conventionally, there is known a developing device of the two-component development type, in which a carrier and a toner that is mixed in a given ratio are agitated, charged, and supplied to an electrostatic latent image that is formed on the outer peripheral surface of a photoconductor drum for use as an image carrier, and the electrostatic latent image is developed with the charged toner.
The developing device of this type comprises a magnet roller having a plurality of magnetic poles arranged along its outer peripheral surface and a nonmagnetic cylindrical developing sleeve that is located around the magnet roller with a given gap. The magnet roller is fixedly provided in a predetermined position adjacent to the photoconductor drum so that its main pole faces the photoconductor drum. The developing sleeve is located for rotation in a position at a given distance from the outer peripheral surface of the photoconductor drum with a given developing gap between them.
A developing agent is held on the outer peripheral surface of the developing sleeve by means of a magnetic field that is generated by the magnet roller, whereupon a magnetic brush based on the developing agent is formed on the outer peripheral surface. The magnetic brush formed in this manner is trimmed to a given length by means of a doctor blade that is opposed to the outer peripheral surface of the developing sleeve, and is transported to the developing gap opposite the outer peripheral surface of the photoconductor drum as the developing sleeve rotates. The toner of the magnetic brush is supplied to the electrostatic latent image by means of a developing bias that is applied between the developing sleeve and the photoconductor drum, whereupon the electrostatic latent image is visualized.
In the conventional developing device of the two-component development type described above, however, a magnetic force (development inhibiting force) that settles a threshold value for development gradually changes in the developing gap where the developing sleeve faces the photoconductor drum, and the density of the magnetic brush that is formed on the developing sleeve according to the distribution of the magnetic poles of the magnet roller is subject to unevenness. Accordingly, the conventional developing device has a problem involving a phenomenon (hereinafter referred to as "tailing") such that the toner floating on the lower-stream side of the developing gap is attracted to the outer peripheral surface of the photoconductor drum in a magnetic trough (low-magnetism portion) between the main pole of the magnet roller and the magnetic pole that is situated directly on the lower-stream side of the main pole.